Cylindrical wavefront

for a linear source in three dimensions, the wavefront are cylindrical.

and intensity, I is inversely proportional to r ( distance from the source )

is the fact given correct??

here's the question.
cylindrical wavefronts are emitted by a linear source of length 4.0 m, at a rate of 50 microwatt.
what is the intensity of this wave at a distance of 60m from the source? i just want to check out whether the answers in the book is correct or not..